Tap switch

ABSTRACT

The invention relates to a tap changer for voltage regulation with semiconductor switching components on a regulating transformer having a regulating winding. The tap changer is of a modular design and each module comprises, respectively, one partial winding of the regulating winding that is connectable or disconnectable by means of semiconductor switching components. Varistors and capacitors are additionally integrated for protecting the semiconductor switching components against high voltages.

The invention relates to a tap changer for voltage regulation with semiconductor switching components.

A regulatable transformer with semiconductor switching components is already described in DE 22 48 166. The secondary winding there comprises a specific number of regulating winding parts, a certain number of which are assembled in a series-connected group of windings, wherein each winding group has two or three regulating winding parts that are connected in parallel. In that instance, each winding part is provided with a contactless switching component. The patent specification also describes another variant wherein the secondary winding of the transformer comprises a group of series-connected regulating winding parts, with each winding part comprising four contactless switching components. The arrangement is equipped such that terminal voltage polarity of the regulating winding part is reversible and the entire regulating winding part is optionally bypassable as well.

A further apparatus for stepped switching of the secondary voltage of a transformer is known from DE 25 08 013 [U.S. Pat. No. 3,978,395]. The secondary winding is grouped in partial windings in that case too, and semiconductor switching components can also be provided.

DE 197 47 712 describes a similar type of tap changer arranged on a tapped transformer that is constructed as an autotransformer. Individual winding parts that are connectable individually and independently from each other, are also provided in this instance. Besides fixed taps of the regulating winding, this arrangement additionally comprises separate switchable or connectable winding parts.

Known from WO 1995/027931 [U.S. Pat. No. 5,604,423] are various embodiments of another tap changer for uninterrupted load transfer, wherein thyristors also serve as switching components. Different winding parts of a tap winding can then be connected or disconnected as part of the secondary winding of the respective tapped transformer by means of anti-parallel connected thyristor pairs. The specification further proposes a method termed “discrete circle modulation” for realizing as fine-stepped a voltage regulation as possible with a limited number of winding taps present, in which method the thyristors are activated in such a manner that intermediate values of the secondary voltage result.

A modularly constructed semiconductor tap changer system is known from the publication “A new approach to solid-state on load tap changing transformers” (Osman, Demirci; David, A., Torrey; Rober, C., Degeneff; IEEE Transactions on Power Delivery, Vol. 13, Issue: 3, July 1998). There, the regulating winding is connected in series with multiple modules. Each of the modules comprises semiconductor switching components and partial windings with different numbers of turns. Targeted actuation of the semiconductor switching components allows the partial windings to be connected with or disconnected from the regulating winding. By the different numbers of turns it is even possible to switch partial windings against each other and against the main winding. This publication constitutes the prior art portion of the first claim.

In the solutions known from the prior art, semiconductor switching components de facto take over the function of the mechanical selector arm in classic mechanical tap changers. Individual winding taps of the regulating windings can specifically be connected or disconnected by means of the semiconductor switching components. It is also possible to subdivide the regulating winding into partial windings that are separately connectable.

These tap changers must comply with the requirements of the different IEC standards 60214-1 in combination with IEC 60060. Among others, a separate source AC withstand voltage test and a lightning impulse voltage test have to be performed. In the separate source AC withstand voltage test, a single-phase alternating voltage of a frequency of 50 or 60 Hz is applied for 60 seconds. According to the approved medium operating voltage, these values can range from 20 kV to 325 kV. The lightning impulse voltage test must be performed at the standard 1.2/50 μs. The alternating voltage can rise up to a level of 1.8 V in the process. Since these alternating voltages apply immediately at the tap changer, in particular at the semiconductor switching components, and since it is either not possible or only possible with great effort to design the semiconductor switching components for such high alternating voltages, damages to the semiconductor switching components are likely to occur.

It is therefore the object of the invention to provide a tap changer with semiconductor switching components wherein the semiconductor components are protected from high alternating voltages resulting from the tests performed in compliance with the standards.

This object is attained by a tap changer having the features of claim 1. The subclaims relate to particularly advantageous developments of the invention.

The general inventive idea consists in connecting in parallel with each semiconductor switching component a voltage-dependent resistor that is series-connected with a capacitor.

The invention will be explained exemplarily in more detail in the following by means of the figure.

The FIGURE shows a transformer for being regulated comprising a low voltage winding 1 and a high voltage winding regulating winding (high voltage winding) 2, here with three separate partial windings W1, W2, and W3, to which a tap changer 3 according to the invention is switched. The dotted line here symbolizes the range of the tap changer 3 that comprises three individual modules M1, M2, M3. The number of modules is, however, selectable as required. The first module M1 comprises the first partial winding W1 and on both sides thereof two bypass paths that comprise a respective series connection of two semiconductor switching components, for example S1.1 and S1.2 or S1.3 and S1.4.

Provided between the respective two serially connected switching components is a center tap M1.1 and M1.2. Arranged in parallel with each semiconductor switching component S1.1-S1.4 are a capacitor C1.1-C1.4 with a series-connected voltage-dependent resistor R1.1-R1.4.

The individual semiconductor switching components are illustrated only schematically as simple switches. In practice, they comprise parallel-connected thyristors, IGBTs or other semiconductor switching components. They can also each comprise a series connection or a parallel connection of a plurality of such individual semiconductor switching components. The voltage-dependent resistors are so-called varistors or transient-voltage-suppression diodes.

The center tap M1.1 of the first module M1 is electrically connected with the one end of the high voltage winding 2. The modules M1 and M2 are electrically connected via the center taps M1.2 and M2.2. This second module M2 is identically constructed; it also comprises a partial winding W2, the two series connections that are each of two semiconductor switching components S2.1 and S2.2 or S2.3 and S2.4, and in parallel therewith capacitors C2.1-C2.4 and resistors R2.1-R2.4 that are connected in series. Again, center taps M2.1 and M2.2 are provided between the respective series connections. The connection of the one center tap M2.2 with the first module M1 has already been explained above; the second center tap M2.1 is for its part is connected with a center tap M3.1 of a third module M3 of the same type. This module also contains semiconductor switching components S3.1-S3.4 with the center taps M3.1 and M3.2 arranged between them, as well as capacitors C3.1-C3.4, resistors R3.1-R3.4 and a partial winding W3. The third module is connected via the center tap M3.2 with a load dissipation (star point) 4.

The three described modules M1, M2, M3 can differ in the dimensions of the respective partial windings W1, W2, W3.

Targeted actuation of the semiconductor switching components Sn.1-Sn.4 of the individual modules M1-M3 allows connecting individual partial windings W1-W3 in the same sense or in the opposite sense. If the partial windings have different dimensions, it is even possible to switch the partial windings against each other.

Performing the tests according to the IEC standard involves the application of very high voltages to the semiconductor switching components Sn.n. In order to protect the semiconductor switching components Sn.n during the lightning impulse voltage test and during the separate source AC withstand voltage test, a voltage-dependent resistor Rn.n (for instance a varistor or a gas-filled surge protector etc.) is additionally connected in parallel.

The voltage-dependent resistor Rn.n is designed to precisely not conduct in the instance of the separate source AC withstand voltage test, or, in a further embodiment, to conduct such a low current that the resulting power dissipation for the time period mentioned will not lead to destroying the components. Under conditions of the lightning impulse voltage test, the voltage-dependent resistor Rn.n becomes conductive and limits the voltage rise across at the semiconductor switching component Sn.n. It is therefore necessary to design the semiconductor switching component Sn.n to withstand the applied voltage without being destroyed.

Compared with the separate source AC withstand voltage test, the voltage pulse rise of the lightning impulse voltage test is many times more rapid, so it also exhibits higher frequencies (greater than 60 Hz) than the range of the separate source AC withstand voltage test (maximally 60 Hz). For this reason, the voltage-dependent resistor Rn.n is linked capacitively. This is realized by the series connection with the capacitor Cn.n. The said link creates a high pass. During the low frequency separate source AC withstand voltage test, the stress is kept away by the voltage-dependent resistor Rn.n.

A particularly advantageous feature of this arrangement lies in the fact that the voltage-dependent resistors Rn.n (varistors) in connection with the capacitors Cn.n thus only have to be designed to the requirements of the lightning impulse voltage test. These requirements are significantly lower than those of the separate source AC withstand voltage test because less energy is contained that is reflected in the costs and the required installation space. A further positive effect is that the semiconductor switching components Sn.n can be designed to smaller dimensions. This also leads to cost benefits. 

1. A tap changer for voltage regulation with semiconductor switching components on a regulating transformer having a regulating winding wherein the tap changer has at least one module, each module comprises a respective partial winding of the regulating winding and on both sides thereof two bypass paths, each bypass path comprises one series connection of two semiconductor switching components one center tap is provided between each two serially connected semiconductor switching components of each bypass path, one of the two center taps of each module is connected with a center tap of the adjacent modules, respectively, the one remaining center tap of the third module is connectable with the load dissipation and the other remaining center tap of the first module is connectable with the regulating winding of the regulating transformer, and a voltage-dependent resistor connected in series with a capacitor is provided in parallel to each semiconductor switching component.
 2. The tap changer according to claim 1, wherein the tap changer consists of exactly three modules.
 3. The tap changer according to claim 2, wherein partial windings of the three modules are divided in the ratio of 1:3:6 or 1:3:9.
 4. The tap changer according to claim 1, wherein the semiconductor switching components each comprise a nonparallel connected thyristor pair or an anti-serially connected IGBT pair.
 5. The tap changer according to claim 1, wherein at least one of the semiconductor switching components comprises a series connection or a parallel connection of at least two individual semiconductor switching components. 